Re: [Xen-devel] [PATCH 4/5] x86/pv: Drop support for paging out the LDT

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.01.18 at 19:37, <andrew.cooper3@xxxxxxxxxx> wrote:
> Windows is the only OS which pages out kernel datastructures, so chances are
> good that this is a vestigial remnant of the PV Windows XP experiment.

This is based on what? How do you know there are no other OSes
doing so, including perhaps ones which none of us has ever heard
of? The diffstat of the change here is certainly nice, but as always
with something being removed from the ABI I'm rather hesitant.

> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -1942,11 +1942,8 @@ int domain_relinquish_resources(struct domain *d)
>          {
>              for_each_vcpu ( d, v )
>              {
> -                /*
> -                 * Relinquish GDT mappings. No need for explicit unmapping of
> -                 * the LDT as it automatically gets squashed with the guest
> -                 * mappings.
> -                 */
> +                /* Relinquish GDT/LDT mappings. */
> +                pv_destroy_ldt(v);
>                  pv_destroy_gdt(v);

The domain is dead at this point, so the order doesn't matter much,
but strictly speaking you should destroy the GDT before destroying
the LDT (just like LDT _loads_ always need to come _after_ GDT
adjustments).

Everything else here looks fine, but the initial comment may need
further discussion. For example we may want to consider a
two-stage phasing out of the feature, with a couple of years in
between: Make the functionality dependent upon a default-off
command line option for the time being, and issue a bright warning
when someone actually enables it (telling them to tell us).

Jan


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